1. Field of the Invention
The present invention relates to a free piston Stirling engine, and more particularly to a free piston Stirling engine for driving a linear generator which has a regulation function of output.
2. Description of the Prior Art
A conventional free piston Stirling engine 13, in general, is provided with following structures as shown in FIG. 1 and operates as follows.
A displacer 4 is air-tightly fitted in a cylinder so as to be able to reciprocate and defines an expansion chamber 11 and a compression chamber 12 in the cylinder. The expansion chamber 11 is communicated with the compression chamber 12 via a heater 1, a regenerator 2 and a cooler 3. In an operational sealing area leading from the expansion chamber 11 to the compression chamber 12, the operational gas such as helium is enclosed. The heater 1 is constituted so that it can be heated by solar heat and so on via many layers of heat sealed 5. The displacer 4 is connected with a rod 10 and reciprocates with the rod 10.
A pair of power pistons, namely free pistons 6 are disposed so that the moving direction of free pistons 6 crosses the reciprocation direction of the rod 10. At an one end of each free piston 6, an operational chamber 18 which is communicated with the compression chamber 12 is defined. On the other hand, a buffer chamber 9 is defined by a cover 19 at the other end of each free piston 6. The covers 19 are provided with projection portions at their center portions and each projection portion is slidably inserted in center opening portion of each free piston 6. Thereby, a gas spring 8 is formed in the center opening portion of each free piston 6. Accordingly, each free piston 6 is constituted so as to be able to freely reciprocate between the operational chamber 18 and the buffer chamber 9. Namely, the free pistons 6 are reciprocated by the compressed operational gas which is periodically supplied from the compression chamber 12 to the operational chambers 18. A pair of linear generators 7 are disposed around the free pistons 6 so as to generate electricity by the reciprocation of the free pistons 6.
When the free piston Stirling engine is activated, the rotational torque of a motor (not shown) is transmitted to the rod 10 via a yoke-cam mechanism and then the displacer 4 is forcibly reciprocated. On the other hand, the operational gas is heated by the solar heat and so on supplied to the heater 1 via many layers of heat sealed 5 and then the operational gas with high temperature and high pressure which is heated by the heater 1 is isothermally expanded in the expansion chamber 11. Next, the operational gas from the regenerator 2 is cooled by the cooler 3 and is isothermally compressed in the compression chamber 12. Thereby, the displacer 4 is reciprocated without the rotational torque of a motor. On the other hand, the operational gas which is compressed goes in and out the operational chambers 18 and the free pistons 6 are forcibly reciprocated. Thereby, the linear generators 7 are activated.
The linear generators 7 are comprised of permanent magnets which are fixed to the outer circumferential surface of the other ends of the free pistons 6 and coils which are fixed to housing located around the permanent magnets. Thereby, the electromotive force is generated in the coils by the reciprocation of the free pistons 6 and the requisite output voltage is obtained by means of selectively changing the taps of the coils by a load sensor (not shown).
In fact, as disclosed in U.S. Pat. No. 4,642,547, the output voltage of the linear generator 7 is regulated as follows. The output voltage of the linear generator is detected by a voltage detector and is compared with a standard value of a voltage setting device. Then the current which flows in a false load connected with the output terminal of the linear generator is regulated in response to the result of the comparison and the output voltage which is supplied to the load is pertinently maintained.
A summary of the above described prior voltage regulation is shown below. When the output voltage of the linear generator is larger than the requisite output voltage of the load, the current is flown in the false load and the voltage which is supplied to the load is lowered. Thereby, the load is prevented from experiencing excessive voltage.
In this voltage regulation, however, the difference between the output of the engine and the quantity of the load is uselessly thrown away as the false load. If the number of the taps of coils is increased and then the changes of the taps of the coils are increased, it is possible to reduce the energy which is wasted in the false load. According to this means, however, the operation for changing the taps of the coils increases and therefore the annexed device is necessitated. Accordingly, it is able to prevent the enlargement of the total size of the free piston Stirling engine and furthermore the operational unforeseen accident such as tap failure is caused.